Arthropod-borne viruses are the most important causes of acute encephalitis worldwide. Several members of the alphavirus family of arboviruses cause encephalitis in the New World. Sindbis virus (SV), an alphavirus, causes acute encephalitis in mice. Non-fatal SV infection of the central nervous system (CNS) elicits a well-characterized mononuclear inflammatory response and provides a model system for studying the development of immune responses within the CNS and the mechanisms by which virus is cleared from neural cells. The inter-relationships between B cells, T cells, and monocytes, their mechanisms of entry into the CNS and their contributions to virus clearance from the brain have not been elucidated. We propose to further our understanding of immune responses in the CNS and their role in recovery from alphavirus encephalitis with the following specific aims: 1. To determine the numbers and immunoglobulin isotypes of SV-specific and tetanus toxoid-specific (irrelevant antigen), immunoglobulin-secreting B cells in the brain, blood and spleens of mice during acute SV using a modified enzyme immunoassay to detect single immunoglobulin-secreting B cells. 2. To determine the importance of SV-specific antibody for recovery from acute encephalitis using B cell deficient xid, anti-u-treated "triple deficient" (NIH-3) and scid mice. Mice unable to clear virus will receive passive transfer of monoclonal or polyclonal antibodies and/or purified cell populations. 3. To characterize the local T cell response during acute encephalitis by determining the types, state of activation and expression of adherence proteins of T cells entering the CSF and brain parenchyma. Cells will be typed according to surface antigens using flow cytometry and for lymphokine production using in situ hybridization and immunospot assays. 4. To determine the proportion and time course of appearance of SV-specific T cells in blood, cervical lymph nodes, spleen, CSF and brain during acute encephalitis using a limiting dilution proliferative assay. 5. To determine the importance of CD4 and CD8 T cells for recovery from acute encephalitis using in vivo depletion of specific cell populations prior to infection. 6. To characterize the expression of MHC class I Ag in the CNS during acute alphaviral encephalitis and identify the cells expressing class I using ultrathin cryosections.